The only reason I need not to believe it is because I didn't hear Mattis say it. If you want to believe second hand anti-Trump stories from Democrat propagandists that's your choice, fren.

You don’t just let people attribute borderline-treasonous quotes to you unless you’re trying to hide from them and avoid attention.

This doesn't even make sense, my dude. There is no recording of Mattis saying this so he has nothing to hide from. There are no repercussions for him in any way based on this hearsay story. I doubt he cares one way or another about it.